The invention relates to an ion trap type mass spectrometer and a mass analyzing method thereof.
A mass spectrometer is a highly sensitive and highly precise instrument that can directly mass-analyze a sample and has been widely used in various fields from astrophysics field to bio-technology field.
There are various kinds of mass spectrometers based on different principles of measurement. Among such mass spectrometers, ion trap type mass spectrometers have rapidly become popular because of their compactness and a variety of functions. The original ion trap type mass spectrometer was invented by Dr. Paul in the 1950s. It is disclosed in U.S. Pat. No. 2,939,952. After that, a lot of researchers have improved devices and techniques. For example, a fundamental technique of obtaining mass spectra by an ion trap type mass spectrometer is disclosed in U.S. Pat. No. 4,540,884. Further, U.S. Pat. No. 4,736,101 discloses a mass spectrometry method of applying a supplementary AC voltage and ejecting and detecting ions in resonance. Furthermore, U.S. Pat. No. 5,466,931 discloses a mass spectrometry method of freely ejecting and dissociating ions in an ion trap using that a supplementary AC voltage comprises a plurality of frequency components (noise having a broad frequency spectrum) instead of a single frequency component. This technology uses a resonance of ion secular frequencies and supplementary AC voltages and can eject a lot of ions in resonance at a time. As the purpose of the wide-band noise signal of the invention is to eject ions of a wide range simultaneously, the noises are at an identical voltage. However, the frequency component corresponding to the frequency of an ion to be stored in the ion trap is notched. The ions corresponding to the notch frequency are steadily stored in the ion trap without causing resonance.
In recent years, various ionization methods for chemical analysis such as matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI) have been developed. This has also enabled mass analysis of biomolecules such as proteins and DNAs. Particularly, the electrospray ionization (ESI) method can directly extract stable gaseous ions from a solution of biomolecules which are apt to be decomposed by heat.
In ESI, biomolecules such as proteins, peptides which are digestive decomposition of protein, and DNAs produces multiply-charged ions. A multiply-charged ion has two or more charges (n) per molecule (m). As the mass spectrometer (MS) mass-analyzes ions by the mass-to-charge (m/z) ratio, the MS handles an ion of molecular weight m having n charges as an ion of a mass-to-charge value m/n. For example, the mass-to-charge (m/z) ratio of protein of molecular weight 30,000 having 30 charges is 1,000 (=30,000/30) and the protein can be mass-analyzed as a singly-charged ion of molecular weight 1,000. Therefore this technology has enabled even a small mass spectrometer such as a quadrupole mass spectrometer (QMS) and an ion trap type mass spectrometer to easily mass-analyze proteins whose molecular weight is over 10,000.
For mass-analysis of a very small amount of components in blood or biological tissue, it is required to remove a lot of interface components (impurities) or to clean up before the mass-analysis.
This clean-up requires lots of time and man-power. However, it is impossible to remove all impurities even by a complicated pre-processing. These impurities disturb the signals of the components of the biological sample. This obstruction is called a chemical noise. To remove or separate such impurities, a liquid chromatography-mass spectrometer (LC/MS) has been developed which comprises a combination of a liquid chromatography (LC) and a mass spectrometer placed before the LC. FIG. 25 shows the schematic diagram of a conventional LC/MS. The mobile phase 32 (a sample solution) of the LC is pumped into an analysis column 35 through an injection port 34 by a pump 33. The analysis column 35 separates impurities from the sample solution (biological sample components) and sends the sample solution to the ESI ion source 36 on-line. The sample solution eluted from the LC is introduced into a spray capillary 37 to which a high voltage is applied in the ESI ion source 36. The sample solution is sprayed from the tip of the capillary 37 into the atmosphere in the ESI ion source 36 to be fine charged droplets (xe2x88x92xcexcm). The fine charged droplets collide with atmospheric molecules in the ESI ion source 36 and are mechanically pulverized into smaller droplets. This collision and pulverization step is repeated until ions are finally ejected into atmosphere. This is the process of electrospray ionization (ESI). The ions are introduced into a mass spectrometer 40 through an intermediate pressure chamber 38 and a high-vacuum chamber 39 which are vacuumed by vacuum pumps 30 and 31 and mass-analyzed there. The result of analysis is given as a mass spectrum by a data processor 41.
The high-sensitivity analysis of extremely trace biological components in blood or tissue cannot be attained easily even by means of pre-processing, cleaning up, and a liquid chromatography (LC). This is because the quantity of a sample to be mass-analyzed is extremely small (10xe2x88x9212 gram or less) and the overwhelming majority of the sample consists of interferences which cannot be fully separated or removed even by preprocessing or the liquid chromatography (LC).
As one of means for solving such problems, U.S. Pat. No. 6,166,378 presents a try to discriminating target components from such interferences components in mass-analysis. Most of interferences in a biological sample are lipids, carbohydrates, and so on whose molecular weight is comparatively low (1,000 or less). These low-molecular-weight components interfere, on the mass spectrum, with bimolecules such as proteins, peptides, and DNAs whose molecular weight is 2,000 or more. This is because the biomolecules give multiply-charged ions and mass peaks appear in a low mass region. In the ESI technology, most of interferences whose molecular weight is comparatively low produce singly-charged ions. Contrarily, most of biomolecules such as proteins and peptides produce multiply-charged ions by the ESI.
Singly-charged ions can be distinguished from multiply-charged ions by accelerating these ions together at a pressure of about 1 torr. By this acceleration, ions repeatedly collide with gas molecules. In this case, if the proton affinity (PA) of the gas molecule is greater than that of the ions, a proton is deprived of the ion and as the result, the ion loses one charge. The multiply-charged ions are apt to cause this ion-molecule reaction and easily transfer protons to neutral molecules such as water. Contrarily, as the ions have fewer charges, this ion-molecule reaction occurs comparatively less. In other words, singly-charged ions are hard to lose charges but multiply-charged ions are apt to lose charges.
U.S. Pat. No. 6,166,378 uses this difference in the ion-molecule reaction and a tandem mass spectrometer which combines three mass spectrometers in tandem to identify mass signals on mass spectrum.
The try to use a tandem mass spectrometer to distinguish singly-charged ions from multiply-charged ions has various problems. One of the problems is that only small part of ions introduced into the tandem mass spectrometer reaches the detector. In other words, the transmission efficiency of ions of the tandem mass spectrometer is very low (xe2x88x92%). Therefore, the measuring sensitivity of tandem mass spectrometer is much lower than the measuring sensitivity that is required by the mass-analysis of biomolecular compounds. Another problem is that the discrimination of singly-charged and multiply-charged ions, that is, the cooperating sweeping of the first and third mass spectrometers (MSs) in tandem can be done only once for one mass spectrum. Therefore, the filtering effect of the signal-to-noise is limited. Furthermore, this technique requires three mass spectrometers in tandem, which makes the system very expensive.
The present invention has been made to solve such problems and it is an object of this invention to provide an improved mass-analyzing method capable of distinguishing singly-charged and multiply-charged ions by an inexpensive ion trap type mass spectrometer.
In accordance with the above object, there is provided a method of mass analyzing a sample by an ion trap type mass spectrometer which is equipped with a mass analyzing unit having a ring electrode and one pair of end cap electrodes and mass-analyzes by temporarily trapping ions in a three-dimensional quadrupole trapping field. This method comprises a first step of applying a main high frequency voltage to said ring electrode to form a three dimensional quadrupole field, a second step of generating ions in said mass analyzing unit or injecting ions from the outside and trapping ions of a predetermined mass-to-charge ratio range in said mass analyzing unit, a third step of applying a supplementary AC voltage having a plurality of frequency components between said end cap electrodes and scanning the frequency components of said supplementary AC voltage, and a fourth step of scanning said main high frequency voltage and ejecting ions from said mass analyzing unit and detecting thereof.
Further, there is provided a method of mass analyzing a sample by an ion trap type mass spectrometer which is equipped with a mass analyzing unit having a ring electrode and one pair of end cap electrodes and mass-analyzes by temporarily trapping ions in a three-dimensional quadrupole trapping field. This method comprises a first step of applying a main high frequency voltage to said ring electrode to form a three dimensional quadrupole field, a second step of generating ions in said mass analyzing unit or injecting ions from the outside and trapping ions of a predetermined mass-to-charge ratio range in said mass analyzing unit, a third step of applying a supplementary AC voltage having a plurality of frequency components between said end cap electrodes and scanning said main high frequency voltage, and a fourth step of scanning said main high frequency voltage and ejecting ions from said mass analyzing unit and detecting thereof.